Aircraft fleet operators are typically under tremendous pressure to operate their aircraft fleets as efficiently as possible. To achieve desired safety and reliability requirements, however, conservative usage assumptions are generally used to determine the life limits of various aircraft components. Usage credit has long been proposed as a means to extend scheduled maintenance intervals in an effort to reduce maintenance cost. These usage credits are typically determined through evaluation of actual aircraft usage of individual aircraft, which may have wide variation from the “usage as severe as expected” model. While such reliability methods have been used as a means to quantify the safety of individual aircraft, it has been acknowledged that much of the overall fleet safety achieved using current fatigue tolerance methods is due to the conservative nature of usage assumptions.
In one model, proposed usage credit is determined based upon detailed component reliability assessments that produce an absolute characterization of reliability or its complement, unreliability, as a function of service life to determine a life limit that provides acceptable safety. Such absolute assessment models, however, are only as good as the assumptions on the distributions of strength, loads and usage. In addition, variability in these factors, especially loads, is difficult to quantify such that absolute reliability methods, no matter how sophisticated, are difficult to validate. Accordingly, a need has arisen for an improved safety methodology for extending scheduled maintenance intervals of aircraft. A need has also arisen for such an improved safety methodology that allows for its level of impact on fleet reliability to be determined.